1. Field of the Invention
This invention relates to a structure of joining two printed circuit boards at their joint lead patterns electrically, and a process for producing the same.
2. Description of the Prior Art
FIGS. 1A and 1B illustrate a first example of the prior known two printed circuit boards A and B to be joined. The board A has a flexible film base 1 on which is formed an electrical circuit pattern 2 with its leads 2a. The circuit pattern 2 is coated with an electrically insulating film 3 as a protective cover. The other board B has a circuit pattern 5 formed on an epoxy paper, epoxy resin plate or like base 4. This circuit pattern 5 is protected by a solder resist layer 6, but its leads 5a are left uncoated with the layer 6 to allow for later application of a reserve solder layer 7. To join these boards A and B, the board A is turned upside down as shown in FIG. 1A, and its leads 2a are placed on the solder layer 7 in registry with the leads 5a of the latter B as shown in FIG. 1B. Then, a heated soldering iron 8 is pressed on the base film 1 at a point just above each of the pairs of superimposed leads 2a and 5a through the solder layer 7 until the solder is melted.
With this prior known joining method, however, because the application of heat from the soldering iron 8 to the solder layer 7 is indirect, or through the base film 1, it takes a long time to melt the reserve solder layer 7. Also, as the melting of the solder layer 7 is often limited only to a small area just under the point of touch of the iron 8, another problem arises that good soldering between the joint leads 2a and 5a is difficult to achieve. To overcome these problems, a special soldering tool of large scale may be employed in some cases. But an alternative problem is produced in that its handling is troublesome. Besides these soldering problems, the above-described example of the prior art has the basic problem that because the solder layer 7 is in spot-contact to both joint leads 2a and 5a, the joining strength is very weak. If the strength is increased by squeezing the film base 1 with the soldering iron 8, the melted solder layer 7 is extruded from the space between the joint leads 2a and 5a in each pair. In this case, therefore, an additional problem is produced in that an erroneous bridge is formed between the adjacent two pairs of joint leads 2a and 5a.